Reversion Safety tools for overseer and subsystems (#3104)

* guide: reversion safety

* guide: manage reversion safety in subsystems

* add leaf status to ActivatedLeaf

* add an LRU-cache to overseer for staleness detection

* update ActivatedLeaf usages in tests to contain status field

* add variant where missed accidentally

* add some helpers to LeafStatus

* address grumbles

polkadot/node/network/collator-protocol/src/collator_side.rs

@@ -921,7 +921,7 @@ mod tests {
 	use polkadot_subsystem::{
 		jaeger,
 		messages::{RuntimeApiMessage, RuntimeApiRequest},
-		ActiveLeavesUpdate, ActivatedLeaf,
+		ActiveLeavesUpdate, ActivatedLeaf, LeafStatus,
 	};
 	use polkadot_subsystem_testhelpers as test_helpers;
 
@@ -1159,6 +1159,7 @@ mod tests {
 				activated: vec![ActivatedLeaf {
 					hash: test_state.relay_parent,
 					number: 1,
+					status: LeafStatus::Fresh,
 					span: Arc::new(jaeger::Span::Disabled),
 				}].into(),
 				deactivated: [][..].into(),